Cytokines are proteins secreted by a cell of the immune system and transduce a signal to a specific cell. They have various kinds, and many of them are especially associated with immunity and inflammation and also associated with cell growth, differentiation, cell death, wound healing and the like (Curr Opin Cell Biol. 1991 April; 3(2):171-5.).
The janus kinase (JAK) family plays a role in cytokine-dependent regulation of the function of cells associated with growth and immune response. Tyk2 is one of the four kinds of janus kinases (JAK1 (also known as janus kinase 1), JAK2 (also known as janus kinase 2), JAK3 (also known as janus kinase 3) and Tyk2 (also known as tyrosine kinase 2)), and it is known to be involved in signal transduction of cytokines such as IFN(interferon)-α, IFN-β, IL(interleukin)-6, IL-10 family (IL-10, IL-19, IL-20, IL-22, IL-28, IL-29), IL-12, IL-23 and the like (Nature Immunology 10, 356-360 (2009), New York Academy of Science 1246, 34-40 (2011)). These cytokines play an important role in immune response when exist in an appropriate amount. However, excessive production of them is thought to be involved in many autoimmune diseases such as psoriasis, rheumatoid arthritis, inflammatory bowel disease, Sjogren's syndrome, Behcet's disease, multiple sclerosis, systemic lupus erythematosus and the like (Journal of Allergy and Clinical Immunology 127, 3,701-721.e70 (2011), Cytokine & Growth Factor Reviews 19, 41-52 (2008), Invest Ophthalmol Vis Sci. 2008 July; 49(7):3058-3064, Ann Rheum Dis. 2010 July; 69(7):1325-1328). For example, Ustekinumab, which is an anti-IL-12/23 monoclonal antibody, has been approved as a therapeutic drug for moderate to severe psoriasis patient in Europe, and furthermore, clinical trials for various diseases in which the IL-12/23 signaling pathway is suggested to be involved are performed. From the foregoing, a Tyk2 inhibitor is a potential therapeutic drug for various autoimmune diseases (Front Biosci. 2011 Jun. 1; 17:3214-32).
Examples of the compound having a structure similar to the compound described in the present specification include the following compounds.    (1) a compound represented by the following formula:
    (Non-Patent Document 1)    (2) a compound represented by the following formula:
    (Non-Patent Document 2).    (3) a compound represented by the following formula:
    (Non-Patent Document 3).    (4) a compound represented by the following formula:
    (Non-Patent Document 4).    (5) In chemical abstract, the following compounds are registrated.
